The following switches are used by GNAT tools that support project files:
There must be only one -P switch on the command line.
Since the Project Manager parses the project file only after all the switches
on the command line are checked, the order of the switches -P,
-Vpx or -X is not significant.
external(name)when parsing the project file.
If name or value includes a space, then name=value should be put between quotes.
-XOS=NT -X"user=John Doe"
Several -X switches can be used simultaneously. If several -X switches specify the same name, only the last one is used.
An external variable specified with a -X switch takes precedence
over the value of the same name in the environment.